Spectra of operators with fixed imaginary parts
Andrzej Pokrzywa · Proceedings of the American Mathematical Society · 1981
The aim of this paper is to obtain the best bound for the distance between the eigenvalues of a Hermitian matrix $B$ and the real parts of eigenvalues of a matrix $B + iA$, where $A$ is also Hermitian, in the terms of eigenvalues of $A$. A similar problem in infinite-dimensional Hilbert space is also considered.